Rollbocks, sometimes called transporter trailers, are narrow gauge railway trucks or bogies that allow a standard gauge wagon to 'piggyback' on a narrow-gauge line. The Vevey system enables a coupled train of standard gauge wagons to be automatically loaded or rolled onto Rollbocks, so that the train can then continue through a change of gauge. The system uses a pair of narrow gauge (750 or 1,000 mm) rails laid in a pit that is built in the middle of a standard gauge track, which is elevated by about 30 cm. It allows the Rollbock bogies to sit underneath the standard gauge tracks and as the Rollbock train is pulled out of the Rollbock siding each bogie picks up one axle of a standard gauge wagon as it rises out of the Rollbock pit. Thus two Rollböcke are needed for a twin-axle wagon. They were a development of the transporter wagon (Rollwagen), designed to keep cost and weight down by avoiding the need for a complete wagon. History The original invention goes back to the Rollwagen of the Schweizer Maschinenfabrik Winterthur (Swiss Locomotive and Machine Works, Winterthur) or SLM patented in 1880. It is a simple system for light loads that found applications even when the superior Langbein system was invented in 1881 by the Maschinenfabrik Esslingen (Esslingen Engineering Works) named after Paul Langbein being the director of the facilities in Saronno, Italy. The Langbein system uses pairs of claws which are folded up laterally by hand, to form a yoke around the axles of the standard gauge bogies. When the bogies are pulled out of the pit the standard gauge wheels can sink onto the Rollbock to find support. This method enables the Rollbock wagons to traverse curves as sharp as radius and, when fully loaded, they could be moved over narrow gauge tracks at a safe speed of . In 1974 a refinement of the concept was developed for the Yverdon–Ste-Croix railway in Switzerland. Unlike the Langbein system, the Vevey system requires no manual intervention to fix the standard gauge axles over the narrow gauge bogies. In this system, rather than the axle being supported, the wheel flanges of the wagons are cradled in brackets projecting from the side of the Rollbock. This lowers the centre of gravity and increases stability. Many modern Rollbock users have converted to the Vevey System. The Vevey Technologies company was bought by Bombardier